内存表示的向下转型

子类：带抵押贷款的房子 我到处都读到了向下倾斜的爆炸，但不知怎的，我想知道记忆中到底发生了什么。我先在这里输入一段代码：Hello guys,Lets start with the classical:Base class: Asset with NameSubclass: Stock with OwnershipSubclass: House with MortgageI have read everywhere the explination of downcasting but somehow I would like to understand what exactly is happening in memory. I will type here the piece of code first:<pre lang="c#"> House h = new House(); Asset asset2 = h; //Case B The downcast fails. Stock stock2 = (Stock)asset2; 如果B我得到一个无效的强制转换异常：无法将Intro.House类型的对象强制转换为Intro.Stock。简单的探索将是资产2不是股票，是房子，这就是为什么它失败。 在aaset2 = h我们说我们现在有一个类型的对象资产但含量为h。探索是在内存中具有变量a（对象实例资产的引用1）和h（对象实例House的引用2），我们复制引用2，这意味着变量a将包含将指向对象h的引用2。基本上它意味着我们在类型a中包含了h的内容。据我所知，内存中的类表示是对象包含对象metada（对象类型，threding标志和GC标志）和字段值的引用和对象。 第一个问题是，如果我们现在指向一个类型为House的对象，为什么a.Owner抛出异常？这个限制在哪里保存在内存中。 第二个问题与最后一行代码有关，其中asset2根据编译器属于House类型，不能转换为类型Stock。在我的locals变量窗口中，它在asset2上显示的类型为Intro.Asset {Intro.House}。在这种情况下，资产2实际上是指向House的内容，而内容不是正确的类型，实际上并不是对象的类型？ For一个简单的例子，它是一个值类型而不是引用类型 object obj = 1; int k; k =（int）obj; 在这种情况下，向下转换是有效的，因为obj的内容是一个int。但是如何在运行时或作为工作来测试实例的类型？它是否包含在它所包含的字段中？并且由于资产包含一个Montgage，它不能被转换为拥有所有权的股票？ 非常感谢，我希望我能解释一下很清楚。推荐答案你可以做什么取决于班级之间的关系： 如果你如此组织：Exactly what you can do depends on the relationship between the classes:If you have it organised like this:public class Asset { }public class Stock : Asset { public string Ownership { get; set; } }public class House : Asset { public string Mortgage { get; set; } }然后是的，你可以这样做：Then yes, you can do this:House h = new House();Asset asset2 = h; 因为House来自Asset，所以House的任何实例也是资产的实例，就像福特也是一样汽车。 但你不能说：Because a House derives from Asset, so any instance of a House is also an instance of an Asset, in the same way that a Ford is also a Car.But you can't say:House h = new House();Asset asset2 = h;Stock stock2 = (Stock)asset2; 因为房子不是股票 - 它们都来自资产，但它们包含不同的附加信息。 就像说福特是一辆汽车，而法拉利是一辆汽车，因此我的福特是法拉利 如果改变班级的推导，你可以这样做稍微：Because a House is not a Stock - they both derive from Asset, but they contain different additional information.It's like saying "a Ford is a Car, and a Ferrari is a Car, therefore my Ford is a Ferrari"You can do it if you change the derivation of your classes slightly:public class Asset { }public class Stock : Asset { public string Ownership { get; set; } }public class House : Stock { public string Mortgage { get; set; } }现在这样可行：Now this works:House h = new House();Asset asset2 = h;Stock stock2 = (Stock)asset2;因为房子既是股票又是资产。 基类的变量可以包含任何实例派生类（任何Car都是Car，无论是Ford还是Ferarri），系统会在运行时精确地确定它的类型 - 但是在编译时你只能使用适合变量类型的属性和方法，不是内容类型：because a House is both a Stock and an Asset.A variable of the base class can contain an instance of any derived class (any Car is a Car, whether it is a Ford or a Ferarri) and the system sorts out exactly which type it is at run time - but at compile time you can only use the properties and methods appropriate to the variable type, not the content type:House h = new House();Stock s = new Stock();Asset a = h;h.Mortgage = "Company";h.Ownership = "Other Company";s.Ownership = "Another Company";s.Mortgage = "Company"; // ERRORa.Mortgage = "Company"; // ERRORa.Ownership = "Other Company"; // ERROR如果您想要一个简单的答案，请：您在代码示例中演示的内容不是向上转换或向下转换。这是对等投射，原则上无效。因为，如果可以编译它，它将允许访问不存在的成员。我甚至要解释原因吗？应该很明显。 -SAIf you want a simple answer, please: what you demonstrate in your code sample is not up-casting or down-casting. This is peer-casting, which cannot be valid in principle. Because, if it was possible to compile it, it would give an access to non-existing member. Do I even have to explain why? Should be quite obvious.—SAAsset asset2 = h; Stock stock2 = (Stock)asset2;asset2.Mortgage将收到编译器错误，因为变量类型为Asset且无法访问House propreties。 will get a compiler error because the variable type is Asset and it has no access to House propreties.Stock stock2 = (Stock)asset2将得到运行时错误，因为实例类型是House类型。 Bassically asset2是Asset类型的变量，并且具有House类型的实例。 我想我只想弄清楚变量的类型和实例的类型是如何存储在内存中的。 will get a run time error because the instance type is of type House.Bassically asset2 is a variable of type Asset and has an instance of type House.I guess I am just trying to figure it out how the type of the variale and the type of the instance is stored in memory. 这篇关于内存表示的向下转型的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！